I have been upping the insulation in my house piece by piece over the past 20 years. Blown in fiberglass in the attic first, huge change! Batt insulation under the floor (crawl space). Not huge but after I could pad around the tile hall in slippers. My 4th room redo, I knocked out wall for a window and saw the R8 batt. Pulled all the outside wall drywall off and went 3" of rigid foam (plus modern windows). Wow! Did the same upstairs and beefed up the roof insulation prior to re-roofing. Another wow!, especially in terms of that room.
That leaves the 4 downstairs bedrooms. Sadly I didn't do the walls when I did the my room. Didn't know yet. That room is the furthest from the furnace and has the most outside wall. As I improve the rest of the house, the furnace spends more and more time napping. My room gets colder and colder.
Well, the 4th room is now unoccupied so it is its turn. It was the warmest room in the house. Closest to the furnace and has its own duct. Pulled off the drywall and R6 batt. It now has just 1" of R5 foam, fitted tight. I blocked the heat duct with a wad of upholstery foam. Late this afternoon I opened the windows and did an air cleaning of the entire house (with the heat shut down0. Closed the windows and left for several hours, forgetting to turn the heat back on. Sitting here at my study just now, I realized it was getting a little cool and "oh yeah! I forgot the heat! (It's about 38F outside. Went to the thermostat. 67F in the hall with no heat for 5 hours! Wow!
To achieve the same effect before the furnace would have kicked on for probably 5 cycles. That room was not a lot colder with no heat than it was before with lots. And that's with just one layer of R5 foam. Tight fitted riigid foam is SO much warmer than standard stapled in batt. Yes, radically more work to fit. But you only do it once.
Next summer, I'll have cellulose blown into the remaining walls and call it good. If and when i do major work in those rooms. I'll pull the drywall and do it right.
I still heat with gas and a little electricity. But despite price increases I pay far less than I did 20 years ago. And the house is much more comfortable (except my bedroom